DC traction motors used to drive wheels on locomotives are traditionally series connected, meaning that the field of the traction motor is in series with the armature of the traction motor. A series connected DC motor exhibits advantageous torque characteristics at low speeds, enabling a locomotive to pull a heavy train from a dead stop. An undesirable characteristic of a series connected DC motor is that if the wheel it is coupled to starts slipping and increasing speed under lower load conditions, the resultant combination of higher armature voltage and lower field current can result in uncontrolled continuous increases in motor speed. Unchecked, this can result in a severely damaged or destroyed traction motor. There are several generations of wheelslip control mechanisms that have been developed to prevent such an occurrence, with each generation of control mechanism adding finer control, thus allowing greater protection and providing higher tractive effort. The latest generation utilizes complex mechanical and electrical sensing techniques that individually control each traction motor using power electronics. The use of power electronics can be expensive to build, expensive and complex to troubleshoot and maintain, and consumes a great deal of the limited space available on the locomotive.
It is an object of the following to address the above-noted disadvantages.